Dual polarization-quadrature phase shift keying modulation (hereinafter, referred to as “DP-QPSK modulation”.) is one of modulation schemes used in a multilevel phase modulation digital coherent transmission technique. Since it has high spectrum utilization efficiency, DP-QPSK modulation is widely employed as a modulation scheme for trunk communication.
FIG. 13 is a block diagram of a general optical transmitter 900 employing the DP-QPSK modulation. In FIG. 13, only components relevant to an optical system of the optical transmitter 900 are described.
A light source 901 is, for example, a semiconductor laser. Continuous light output from the light source 901 is branched into two rays by a spectroscope 902. The branched continuous light rays are phase-modulated by modulators 903 and 904, and become signal light rays. A polarization plane of the signal light ray output from the modulator 903 is 90 degrees rotated by a ½ wavelength plate (λ/2 plate) 905. The signal light ray emitted from the modulator 904 is transmitted through a delay plate 906 for matching a phase with the signal light ray passed through the ½ wavelength plate 905. Polarizations of these signal light rays are multiplexed by a polarization multiplexer 907. The multiplexed signal light transmitted through a spectroscope 913 is coupled to an optical fiber 908.
Parts of signal light rays are reflected by spectroscopes 911 to 913 and are coupled to monitor PDs (photo diodes, light-receiving elements) 921 to 923, in order to monitor optical intensities of the signal light rays. The spectroscopes 911 to 913 are prisms that include reflection films.
FIG. 14 is a diagram illustrating a specific arrangement of optical components constituting the optical system of the optical transmitter 900. The optical components including prisms and lenses used in the optical transmitter 900 constitute an optical module 990. The optical components described in FIG. 14 are all independent components, and are mounted individually.
In FIG. 13 and FIG. 14, elements having the same functions are assigned with the same reference numerals. Correspondence of the optical components described in FIG. 13 to the optical components described in FIG. 14 is as follows. The spectroscope 902 includes a 50% reflection prism 9021 and a 100% reflection prism 9022. The modulator 903 includes lenses 9031 and 9033, and a modulator 9032. The modulator 904 includes lenses 9041 and 9043, and a modulator 9042. 5% reflection prisms 931 to 933 correspond to the spectroscopes 911 to 913 in FIG. 13. The polarization multiplexer 907 in FIG. 13 includes a polarization multiplexer 9071 and a 100% reflection prism 9072 in FIG. 14. Light rays transmitted through the prisms are all collimated light rays, and the lenses 9031, 9033, 9041, and 9043 convert the collimated light rays in such a way as to be coupled to the modulators 9032 and 9042.
The 50% reflection prism 9021 receives continuous light input from the light source 901 in FIG. 13. The continuous light is branched into two rays by the 50% reflection prism 9021, and the branched continuous light rays are subjected to phase modulation by the modulators 9032 and 9042. The 5% reflection prisms 931 and 932 reflect 5% of optical power of the respective signal light rays modulated by the modulators 9032 and 9042, and guide the reflected signal light rays to the monitor PDs 921 and 922. The polarization multiplexer 9071 couples the signal light rays transmitted through the 5% reflection prisms 931 and 932, and generates polarization-multiplexed light. The 5% reflection prism 933 reflects 5% of optical power of the polarization-multiplexed signal light, and guides the reflected polarization-multiplexed signal light to the monitor PD 923. The 100% reflection prisms 9022 and 9072 are used for changing a direction of light. The polarization-multiplexed light transmitted through the 5% reflection prism 933 is coupled to the optical fiber 908 in FIG. 13.
In relation to the present invention, each of PTL 1 and PTL 2 discloses a configuration of an optical module that includes an optical filter and a prism.